Carbon dioxide has a large number of uses. For example, carbon dioxide is used to carbonate beverages, to chill, freeze and package seafood, meat, poultry, baked goods, fruits and vegetables, and to extend the shelf-life of dairy products. It is an important environmental component in industrial waste and process water treatment as a replacement for sulfuric acid to control pH levels. Other uses include drinking water treatment, an environmentally-friendly pesticide and an atmosphere additive in greenhouses to improve the growth of vegetables.
Generally carbon dioxide is produced by purifying a waste stream which is a by-product of an organic or inorganic chemical process. The waste stream, which comprises a high concentration of carbon dioxide, is condensed and purified in multiple steps and then distilled to produce the product grade carbon dioxide.
As the demand for carbon dioxide continues to increase, alternate sources of carbon dioxide are being used to supply the crude carbon dioxide feed to the purification system. Such alternate feeds have a much lower concentration of carbon dioxide and thus need to be upgraded, i.e. the concentration of the carbon dioxide must be increased, before product grade carbon dioxide can be effectively produced. These alternate feeds with much lower carbon dioxide concentrations will be referred to as lean feeds. An example of such a lean feed is flue gas that could come from a combustion source such as a boiler, internal combustion engine, gas turbine or lime kiln.
Upgrading of the carbon dioxide concentration in a feed can be carried out in a number of ways. One particularly preferred method is the chemical absorption of carbon dioxide from the crude carbon dioxide feed into an alkanolamine based absorbent. The resulting carbon dioxide loaded absorbent then undergoes separation into carbon dioxide product for recovery and into alkanolamine containing absorbent which is typically recycled for reuse within the recovery system.
Often the crude carbon dioxide feed contains significant levels of oxygen which can cause degradation of the alkanolamines reducing their utility in the recovery system and also causing corrosion problems in the system. Such oxygen could come from the feed itself and/or equipment leaks, or from makeup fluids. Those skilled in the art have addressed this problem in one of two ways. In one method, chemical inhibitors are added to the absorber fluid to protect against degradation by inhibiting the oxidation of the alkanolamines. In another method, a combustible fuel is added to the crude carbon dioxide feed for combustion with the oxygen in a catalytic combustion reaction. While both methods are effective they are both characterized by high capital costs and, moreover, are complicated to operate.
Accordingly, it is an object of this invention to provide a system which can more effectively recover carbon dioxide or other absorbate from an oxygen containing feed using an alkanolamine based absorbent to upgrade the feed.